gh-143804: highlight default empty-line command repetition in cmd.Cmd documentation

Doc/library/cmd.rst

@@ -93,6 +93,18 @@ A :class:`Cmd` instance has the following methods:
    are the beginning and ending indexes of the prefix text, which could be used to
    provide different completion depending upon which position the argument is in.
 
+.. rubric:: Empty input behavior
+
+When the user enters an empty line in response to the prompt, ``Cmd`` does
+**not** ignore the input by default. Instead, it repeats the last non-empty
+command entered.
+
+This behavior is implemented by :meth:`Cmd.emptyline`. Subclasses that do not
+want empty input to repeat the previous command  should override
+:meth:`Cmd.emptyline` to do nothing::
+
+   def emptyline(self):
+       pass
 
 .. method:: Cmd.do_help(arg)
 
@@ -127,6 +139,8 @@ A :class:`Cmd` instance has the following methods:
    Method called on an input line when the command prefix is not recognized. If
    this method is not overridden, it prints an error message and returns.
 
+   Note that if :meth:`emptyline` is not overridden, empty input will cause
+   the previous command to be repeated and passed again to this method.
 
 .. method:: Cmd.completedefault(text, line, begidx, endidx)
 

Misc/NEWS.d/next/Documentation/2026-01-14-00-09-43.gh-issue-143804.KHeDcP.rst

@@ -0,0 +1,2 @@
+Clarified the documentation of :mod:`cmd` regarding the default handling of
+empty input lines.